Most of conventional stainless steel strips used as building exterior materials where atmospheric corrosion resistance is required are of relatively small areas as typified by panels, sashes and curtain walls.
However, the demand for stainless steel as large sized building exterior materials as typified by roofing members is expanding in the recent years because of the appreciated design, aesthetic appearance, corrosion resistance and weathering features inherent to stainless steel, combined with the development of its application technique. In this regard, painted stainless steel strips based on stainless steel strips coated with paint for coloring are mainly used as roofing members, for example. This is intended to overcome the drawback of conventional zinc coated steel roofing members which yield painting defects due to deterioration of the paint coating and eventually become unusable. For the painted stainless steel strips, SUS 304 (18Cr-8Ni), which is an austenitic stainless steel, has been employed from the standpoint of its workability. However, because of the paint coating on the surface, the painted stainless steel strips cannot take advantage of the design feature offered by the metallic luster of silver white color inherent to stainless steel. Another consideration is the increased cost of austenitic stainless steel which contains more of expensive nickel.
In addition, austenitic stainless steel is not suitable as a length of steel since its coefficient of thermal expansion is about 1.5 to 2 times that of ferritic stainless steel. Ferritic stainless steel thus seems more attractive as building exterior materials although it is inevitably required to have the satisfactory outdoor atmospheric corrosion resistance and rust prevention ability that red rust and corrosion such as pitting corrosion caused by deposition of sea-salt fine particles do not occur over a long term, when used as building exterior materials, especially as roofing members without painting. For this and other reasons, targeting the high atmospheric corrosion resistance, high rust prevention ferritic stainless steel, attempts have been made to enhance corrosion resistance by reducing C and N contents, increasing Cr content and increasing the amount of Mo added, as typified by Japanese Patent Application Kokai No. 138058/1980. However, simply increasing the Cr content and the amount of Mo added results in a high alloy having an increased cost and hence a limit from an economical aspect, and there arise problems from a loss of shapability due to increased hardness and a loss of productivity due to decreased toughness. Thus there is a strong desire to develop a material which can be expected to be increased in atmospheric corrosion resistance and rust prevention by adding elements other than Cr and Mo and which is of lower cost.
On the other hand, in the event that steel is used in areas where it must be welded for a construction reason, conventional high Cr content steel and Mo-containing steel are insufficient in the toughness and rust prevention of welds.